1. Field of the Invention
The present invention relates to a night vision device for a vehicle such as a motor vehicle.
2. Description of Related Art
It has been proposed previously to provide a night vision device in a vehicle such as an automobile. The purpose of the night vision device is to enhance the vision of the driver in night conditions.
Various night vision devices have been proposed before. U.S. Pat. No. 5,414,439 describes a night vision arrangement which utilises a “head-up” display. A motor vehicle is provided with an infra-red camera positioned to view the roadway in front of the vehicle. The camera generates a video signal which is passed to a “head-up” display of the type utilised in combat aircraft. In the embodiment described, the windshield of the motor vehicle or a semi-transmitter is utilised as a combiner to combine the image of the road ahead, as viewed through the windshield by the driver, and a virtual image of the road ahead from an image generator which receives the video signal from the camera.
In U.S. Pat. No. 5,414,439 the infra-red camera is mounted in a fixed position to provide an image of the view in the front of the vehicle. Also the camera has a fixed focal length. Consequently, when cornering the vehicle, that is to say when turning the vehicle to the right or to the left, the camera will always point in a direction aligned with the longitudinal axis of the vehicle. However, there is a need for the driver of the vehicle to view the environment into which the vehicle is moving which will be located either to the left or to the right of the fixed field of view of the camera.
JP-A-0 6048247 discloses a vehicle mounted infra-red image display arrangement, and in this arrangement the precise field of view that is displayed to the driver of the vehicle is dependent upon the position of the front or steering wheels of the vehicle. However, even in an arrangement such as this, it is difficult to ensure that the appropriate image is displayed, since the field of view that is of interest to the driver is not necessarily aligned with the rolling direction of the steering wheels of the vehicle.
When a motor vehicle is being driven quickly, the driver of the vehicle tends to concentrate on the road ahead of the vehicle, but, in contrast, when the vehicle is travelling slowly, for example in a built-up area, the driver usually pays attention to situations that may develop on either side of the roadway. Thus the driver may be particularly conscious of pedestrians who are not in the roadway, but who may step into the roadway in front of the vehicle. Thus, when a driver is driving quickly, the driver tends to concentrate on a relatively narrow angular field of view, whereas when the vehicle is driving more slowly, the driver tends to concentrate on a wider angular field of view. If the image provided by a night vision arrangement of the type discussed above were to be utilised to provide the wide field of view, whilst providing a sufficiently large image of the road ahead for fast driving, with a fixed focal length of the lens of the infra-red camera, a very wide display would have to be provided, which may prove to be impractical in many vehicle installations.